(Opto)electronic arrangements are being used with ever-increasing frequency in commercial products or are close to market introduction. Such arrangements comprise organic or inorganic electronic structures, examples being organic, organometallic or polymeric semiconductors or else combinations of these. Depending on the desired application, these arrangements and products are rigid or flexible in form, there being an increasing demand for flexible arrangements. Arrangements of this kind are produced, for example, by printing techniques, such as relief, gravure, screen or planographic printing, or else what is called “non-impact printing”, such as, for instance, thermal transfer printing, inkjet printing or digital printing. In many cases, however, vacuum techniques are used as well, such as chemical vapour deposition (CVD), physical vapour deposition (PVD), plasma-enhanced chemical or physical deposition techniques (PECVD), sputtering, (plasma) etching or vapour coating, with patterning taking place generally through masks.
Examples of (opto)electronic applications that are already commercial or are of interest in terms of their market potential include electrophoretic or electrochromic constructions or displays, organic or polymeric light-emitting diodes (OLEDs or PLEDs) in readout and display devices, or as illumination, electroluminescent lamps, light-emitting electrochemical cells (LEECs), organic solar cells, preferably dye or polymer solar cells, inorganic solar cells, preferably thin-film solar cells, more particularly those based on silicon, germanium, copper, indium and selenium, organic field-effect transistors, organic switching elements, organic optical amplifiers, organic laser diodes, organic or inorganic sensors or else organic- or inorganic-based RFID transponders.
A perceived technical challenge for realization of a sufficient lifetime and function of (opto)electronic arrangements in the area of organic and/or inorganic (opto)electronics is the protection of the components they contain against permeants. Permeants may be a large number of low molecular mass organic or inorganic compounds, more particularly water vapour and oxygen.
A large number of (opto)electronic arrangements in the area of organic and/or inorganic (opto)electronics, especially where organic raw materials are used, are sensitive not only to water vapour but also to oxygen, and for many arrangements the penetration of water vapour is classed as a relatively severe problem. During the lifetime of the electronic arrangement, therefore, it requires protection by means of encapsulation, since otherwise the performance drops off over the period of application. For example, oxidation of the components, in the case of light-emitting arrangements such as electroluminescent lamps (EL lamps) or organic light-emitting diodes (OLEDs) for instance, may drastically reduce the luminosity, the contrast in the case of electrophoretic displays (EP displays), or the efficiency in the case of solar cells, within a very short time.
In organic and/or inorganic (opto)electronics, particularly in the case of organic (opto)electronics, there is a particular need for flexible bonding solutions which constitute a permeation barrier to permeants, such as oxygen and/or water vapour. In addition there are a host of further requirements for such (opto)electronic arrangements. The flexible bonding solutions are therefore intended not only to achieve effective adhesion between two substrates, but also, in addition, to fulfil properties such as high shear strength and peel strength, chemical stability, ageing resistance, high transparency, ease of processing, and also high flexibility and pliability.
One approach common in the prior art, therefore, is to place the electronic arrangement between two substrates that are impermeable to water vapour and oxygen. This is then followed by sealing at the edges. For non-flexible constructions, glass or metal substrates are used, which offer a high permeation barrier but are very susceptible to mechanical loads. Furthermore, these substrates give rise to a relatively high thickness of the arrangement as a whole. In the case of metal substrates, moreover, there is no transparency. For flexible arrangements, in contrast, planar substrates are used, such as transparent or non-transparent films, which may have a multi-ply configuration. In this case is it possible to use not only combinations of different polymers, but also organic or inorganic layers. The use of such planar substrates allows a flexible, extremely thin construction. For the different applications there are a very wide variety of possible substrates, such as films, wovens, nonwovens and papers or combinations thereof, for example.
In order to obtain the most effective sealing, specific barrier adhesives are used. A good adhesive for the sealing of (opto)electronic components has a low permeability for oxygen and particularly for water vapour, has sufficient adhesion to the arrangement, and is able to flow well onto the arrangement. Low adhesion to the arrangement reduces the barrier effect at the interface, so permitting the ingress of oxygen and water vapour independently of the properties of the adhesive. Only if the contact between adhesive and substrate is continuous are the properties of the adhesive the determining factor for the barrier effect of the adhesive.
For the purpose of characterizing the barrier effect it is usual to state the oxygen transmission rate OTR and the water vapour transmission rate WVTR. Each of these rates indicates the flow of oxygen or water vapour, respectively, through a film per unit area and unit time, under specific conditions of temperature and partial pressure and also, optionally, further measurement conditions such as relative atmospheric humidity.
The lower the values the more suitable the respective material for encapsulation. The statement of the permeation is not based solely on the values of WVTR or OTR, but instead also always includes an indication of the average path length of the permeation, such as the thickness of the material, for example, or a standardization to a particular path length.
The permeability P is a measure of the perviousness of a body for gases and/or liquids. A low P values denotes a good barrier effect. The permeability P is a specific value for a defined material and a defined permeant under steady-state conditions and with defined permeation path length, partial pressure and temperature. The permeability P is the product of diffusion term D and solubility term S P=D*S
The solubility term S describes in the present case the affinity of the barrier adhesive for the permeant. In the case of water vapour, for example, a low value for S is achieved by hydrophobic materials. The diffusion term D is a measure of the mobility of the permeant in the barrier material, and is directly dependent on properties, such as the molecular mobility or the free volume. Often, in the case of highly crosslinked or highly crystalline materials, relatively low values are obtained for D. Highly crystalline materials, however, are generally less transparent, and greater crosslinking results in a lower flexibility. The permeability P typically rises with an increase in the molecular mobility, as for instance when the temperature is raised or the glass transition point is exceeded.
Approaches at increasing the barrier effect of an adhesive must take particular account of the two parameters D and S, with a view to their influence on the permeability of water vapour and oxygen. In addition to these chemical properties, thought must also be given to consequences of physical effects on the permeability, particularly the average permeation path length and interface properties (flow-on behaviour of the adhesive, adhesion). The ideal barrier adhesive has low D values and S values in conjunction with very good adhesion to the substrate.
A low solubility term S is usually insufficient for achieving good barrier properties. One classic example of this, in particular, are siloxane elastomers. The materials are extraordinarily hydrophobic (low solubility term), but as a result of their freely rotatable Si—O bond (large diffusion term) have a comparatively low barrier effect for water vapour and oxygen. For a good barrier effect, then, a good balance between solubility term S and diffusion term D is necessary.
For this purpose use has hitherto been made in particular of liquid adhesives and adhesives based on epoxides (WO 98/21287 A1; U.S. Pat. Nos. 4,051,195 A; 4,552,604 A). As a result of a high degree of crosslinking, these adhesives have a low diffusion term D. Their principal field of use is in the edge bonding of rigid arrangements, but also moderately flexible arrangements. Curing takes place thermally or by means of UV radiation. Full-area bonding is hard to achieve, owing to the contraction that occurs as a result of curing, since in the course of curing there are stresses between adhesive and substrate that may in turn lead to delamination.
Using these liquid adhesives harbours a series of disadvantages. For instance, low molecular mass constituents (VOCs—volatile organic compounds) may damage the sensitive electronic structures in the arrangement and may hinder production operations. The adhesive must be applied, laboriously, to each individual constituent of the arrangement. The acquisition of expensive dispensers and fixing devices is necessary in order to ensure precise positioning. Moreover, the nature of application prevents a rapid continuous operation, and the laminating step that is subsequently needed may also make it more difficult, owing to the low viscosity, to achieve a defined layer thickness and bond width within narrow limits.
Furthermore, the residual flexibility of such highly crosslinked adhesives after curing is low. In the low temperature range or in the case of 2-component systems, the use of thermally crosslinking systems is limited by the potlife, in other words the processing life until gelling has taken place. In the high temperature range, and particularly in the case of long reaction times, in turn, the sensitive (opto) electronic structures limit the possibility of using such systems—the maximum temperatures that can be employed in the case of (opto)electronic structures are often 60° C., since above even this temperature there may be initial damage. Flexible arrangements which comprise organic electronics and are encapsulated using transparent polymer films or assemblies of polymer films and inorganic layers, in particular, have narrow limits here. The same applies to laminating steps under high pressure. In order to achieve improved durability, it is advantageous here to forego a temperature loading step and to carry out lamination under a relatively low pressure.
As an alternative to the thermally curable liquid adhesives, radiation-curing adhesives are now used in many cases (US 2004/0225025 A1). The use of radiation-curing adhesives prevents long-lasting thermal load on the electronic arrangement. As a result of the irradiation, however, there is short-term pointwise heating of the arrangement, since, in general, there is not only UV radiation emitted but also a very high fraction of IR radiation. Other aforementioned disadvantages of liquid adhesives, such as VOC, contraction, delamination and low flexibility, are likewise retained. Problems may come about as a result of additional volatile constituents or elimination products from the photoinitiators or sensitizers. Moreover, the arrangement must be transparent to UV light.
Since constituents especially of organic electronics, and many of the polymers employed, are frequently sensitive to UV exposure, a long-lasting outdoor use is not possible without further, additional protective measures, such as further outer films, for instance. In the case of UV-curing adhesive systems, these films can be applied only after UV curing, whereby additionally increasing the complexity of the manufacture and the thickness of the arrangement.
US 2006/0100299 A1 discloses a UV-curable pressure-sensitive adhesive tape for encapsulating an electronic arrangement. The pressure-sensitive adhesive tape has an adhesive based on a combination of a polymer having a softening point of more than 60° C., a polymerizable epoxy resin having a softening point of below 30° C., and a photoinitiator. The polymers may be polyurethane, polyisobutylene, polyacrylonitrile, polyvinylidene chloride, poly(meth)acrylate or polyesters, but more particularly may be acrylate. Also present are tackifier resins, plasticizers or fillers.
Acrylate compositions have very good resistance to UV radiation and various chemicals, but possess very different bond strengths to different substrates. Whereas on polar substrates such as glass or metal the bond strength is very high, the bond strength on apolar substrates, such as polyethylene or polypropylene, for example, tends to be low. Here there is a particular risk of diffusion at the interface. Moreover, these compositions are highly polar, which promotes diffusion, particularly of water vapour, in spite of subsequent crosslinking. This tendency is increased further through the use of polymerizable epoxy resins.
The pressure-sensitive adhesive embodiment specified in US 2006/0100299 A1 has the advantage of simplicity of application, but likewise suffers from possible elimination products as a result of the photoinitiators present, from an inevitable UV transparency of the construction, and from a reduction in flexibility after curing. Moreover, as a result of the low fraction of epoxy resins or other crosslinkers that is necessary to maintain the pressure-sensitive adhesion, and more particularly the cohesion, the crosslinking density achievable is very much lower than with liquid adhesives.
In contrast to liquid adhesives, as a result of the relatively high molecular mass polymers, pressure-sensitive adhesive tapes generally require, for effective wetting and adhesion on the surface, a certain time, sufficient pressure and a good balance between viscous component and elastic component. In general the subsequent crosslinking of the adhesives results in shrinkage of the composition. This may lead to a reduction in the interface adhesion, and may in turn increase the permeability.
WO 2007/087281 A1 discloses a transparent flexible pressure-sensitive adhesive tape based on polyisobutylene (FIB) for electronic applications, especially OLEDs. It uses polyisobutylene having a molecular weight of more than 500 000 g/mol and a hydrogenated cyclic resin. An optional possibility is the use of a photopolymerizable resin and a photoinitiator.
The low polarity of adhesives based on polyisobutylene gives them a good barrier to water vapour, but even at high molecular weights they have a relatively low cohesiveness, and so even at room temperature and especially at elevated temperatures a creep tendency may be observed under load, and the adhesives therefore exhibit a low shear strength. The fraction of low molecular mass constituents cannot be reduced ad infinitum, since otherwise the adhesion is significantly lowered and there is an increase in the interface permeation. When a high fraction of functional resins is used, which is necessary on account of the very low cohesion of the composition, the polarity of the composition goes up again and hence the solubility term is increased.
In contrast, a pressure-sensitive adhesive with pronounced crosslinking exhibits good cohesion, but the flow-on behaviour is impaired. The pressure-sensitive adhesive is unable to conform adequately to the roughness of a substrate surface, and this increases the permeation at the interface. Moreover, a pressure-sensitive adhesive with pronounced crosslinking is only able to a relatively small degree to dissipate deformation energy of the kind which occurs under load. Both of these phenomena reduce the bond strength. A pressure-sensitive adhesive with a slight degree of crosslinking, in contrast, is able to flow on well to rough surfaces and to dissipate deformation energy, and hence the adhesion requirements may be met, and yet, owing to reduced cohesion, the pressure-sensitive adhesive does not provide sufficient resistance to a load.
Known from the prior art, furthermore, is a pressure-sensitive adhesive without barrier properties (WO 03/065470 A1), which is used as a transfer adhesive in an electronic construction. The adhesive comprises a functional filler which reacts with oxygen or water vapour within the construction. Consequently, simple application of a scavenger within the construction is possible. The construction is sealed with respect to the outside by using another adhesive with a low permeability.
Known from the prior art, from U.S. Pat. No. 4,985,499 A1, for example, is an adhesive based on vinylaromatic block copolymers. That specification describes various advantageous compositions of the adhesive.
Also known from the prior art is the barrier effect of block copolymers (US 2002/0188053 A1). Here, polymers on that basis are used for the sealing of electrophoretic displays, the active substances being coated, following their application, with a solution of the polymer, this solution, after drying, acting as a sealing layer and fixative. These polymers do not have self-adhesive properties and achieve adhesion to the other components of the electrophoretic display construction only as a result of the wetting from the solution. This entails the use of solvents, produces emissions, and is difficult to meter.
It is an object of the present invention to specify a method of encapsulating an electronic arrangement with respect to permeants, especially water vapour and oxygen, that is simple to carry out and with which at the same time effective encapsulation is achieved. Furthermore, the lifetime of (opto)electronic arrangements is to be increased through the use of a suitable, especially flexible, adhesive.
The present invention solves the problem described above through a method for encapsulating an electronic arrangement with respect to permeants.